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Old 8th June 2012, 02:38 AM   #1
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Default Why no FWB output for SMPS

Does anyone know why a FWB with no center tap is rarely used for a single output with a half or full bridge SMPS. In fact it is not very easy to even buy a ultra fast FWB for SMPS use.
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Old 8th June 2012, 04:47 PM   #2
gmarsh is offline gmarsh  Canada
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Cost/efficiency, I'd imagine.

You're buying twice as many diodes, and 2x the diode drop = 2x the power loss in the output rectifier which means you're buying 2x the heatsinking etc.

Full bridge rectifier output saves a winding in the output transformer, but the full bridge winding has to handle more average current requiring thicker wire or more strands, meaning no copper savings.
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Old 8th June 2012, 05:21 PM   #3
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From my (large) experience repairing SMPSīs of various types, also add that it is relatively rare to have a fault in a transformer, while it is normal to find a shorted diode, so reliability is also reduced when more silicon is added.
Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001.
Argentine Ham Radio LW1DSE since 1987.
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Old 8th June 2012, 05:25 PM   #4
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Adding the extra drop only makes sense at higher output voltages, where you can reap the advantages of a simpler output winding. It also reduces stress on the output diodes - important at higher output voltages. At the usual 5/12/24V levels, it doesn't make a lot of sense.
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Old 10th June 2012, 02:02 AM   #5
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I found this out from the book Transformer and Inductor Design 3rd edition by COLONEL WM. T. MCLYMAN. Chapter 7 -4

The total apparent power into the SMPS is the following.

2x the load power for a FWB to FWB converter.

2.414x the load power for FWB to a FWB CT.

2.828X the load power for a CT input and FWB CT output.

Also from Wikipedia, power factor is the ratio of real power to apparent power.

My conclusion is, without power factor correction a FWB output can have lower primary current. However if you use power factor correction, i am not certain how good the above multipliers can be improved.
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